I have a dimension for "Departments" and another dimension for "Expenses Accounts", I manage the sparsity in a separate cube and use MDX subset as not all departments have all expenses accounts.

In PAX, I want to create two dependant dropdown lists, one for the departments, second for the Expenses accounts that depend on the first one, That is, When a user select a department from the first list, The second list will have only the Expenses accounts associated with this department.